Distributed manufacturing of an open-source tourniquet testing system

Tourniquets are effective for casualty-prevention in emergency situations. The use of centrally-manufactured commercial tourniquets, however, is not always possible due to supply chain disruptions. The open-source hardware model has been applied to overcome these disruptions in humanitarian crises and several low-cost digitally manufacturable open-source tourniquets have been developed. With the low reliability of improvised tourniquets, it is important to ensure that distributed manufacturing of tourniquets is effective and safe. Tourniquets can be tested, but existing tourniquet testers are expensive, bulky, and complex to operate, which limits their accessibility to an even greater extent than tourniquets in extreme settings. This article fulfills a need by providing a small, transportable, open-source additive-manufactured tourniquet tester that enables inexpensive and accurate testing of tourniquets against known clinical parameters. The <$100 tourniquet tester is validated and tested for operating force of tourniquets in the field or in distributed manufacturing facilities. The tourniquet tester has a significant economic and operational advantage compared to proprietary counterparts available on the market. Once calibrated with a blood pressure monitor, the built-in LCD displays the measuring range of the tester as 0 to 200 N, which is enough to test the validation of all tourniquets.

users or tourniquet manufacturers, which has created confusion in validating tourniquet clinical field performance. ASTM has been developing test fixture and tourniquet testing standards since 2016, but these standards are not yet publicly available [64]. Effective tourniquet application requires a combination of end-user training and mechanically effective tourniquet unit performance. The end-user must be confident a tourniquet dependably meets a clinical standard before patient application [65]. Applying a tourniquet incorrectly can cause complications, such as nerve damage, tissue ischemia, or amputation. Therefore, it is important to ensure that tourniquets are effective and safe before using them on patients. See (Fig. 3).
Tourniquet performance can be validated by a tourniquet tester, a device which measures the pressure applied by the tourniquet and the blood flow in the limb [66,67]. Tourniquet testers can help evaluate the quality of different types of tourniquets, such as pneumatic, elastic, or mechanical systems [68]. They can also help train medical personnel on how to use tourniquets properly and monitor their effects [69,70]. Unfortunately, existing tourniquet testers are expensive, bulky, or complex to operate (see Table 1). They may also require specialized equipment or calibration procedures that limit their accessibility and usability in low-resource settings [71]. There is a clear need for an open-source additive manufactured unit tester that would allow a manufacturer or end-user to inexpensively and accurately test their device against a known clinical parameter, an important resource for conflict-affected and resource-limited communities.
To fill this need, this article describes an open-source 3-D printed tourniquet tester that is low-cost, portable, and easy to both manufacture and deploy across diverse scenarios. The design and fabrication process of the device using common materials and tools is described. The tourniquet tester is validated for functionality and accuracy by comparing it with a commercial tourniquet tester on different types of tourniquets. A low-cost calibration process is demonstrated, and potential errors are quantified. The advantages and limitations of the device are discussed and future work is outlined. See (Fig. 4).

Hardware description
An open-source 3-D printable tourniquet tester is developed. All the custom mechanical components can be fabricated with a low-cost desktop fused filament fabrication-based RepRap-class 3-D printer. The electronic components are all open source and readily available off-the-shelf from a wide variety of vendors. The compact design of the tourniquet tester makes it easy to manufacture and easy to transport. The electronic circuit diagrams are provided to power the testing unit via a USB input. The tourniquet tester is validated and tested on Glia units as well as CAT units for testing the operating force of tourniquets in the field or in distributed manufacturing facilities (e.g., fab labs, makerspaces, 3-D print shops, libraries, schools, or in volunteers' homes). As of this writing, there is no agreement on testing standards from the tourniquet manufacturers, so this open-source 3-D printed tourniquet tester provides a means of democratizing the environments which require tourniquet use. The tourniquet tester has a significant economic and operational advantage compared to proprietary counterparts available on the market.
Designed for distributed manufacturing settings and resource-limited communities seeking to validate donated commercial tourniquets, validate novel/generic commercial tourniquets, and those made with distributed manufacturing consistent with the Glia and other types of open-source tourniquets. Low production cost: All the housing components are manufactured by 3-D printing and assembled easily and quickly. Portable and durable: The compact design makes the testing unit easy to transport and can be made from the choice of high-durability 3-D printing materials. Simulated upper arm and shoulder with realistic wounds and interior blood flow. When the tourniquet is correctly applied, hemostasis is achieved on simulated blood flowing from the wound. Does not show data.
Simple operation: Once the tester is calibrated with a blood pressure cuff, it does not need any extra steps or adjustments. The pressure reading is directly shown on the LCD display. According to the measuring range of the load cell (0-20 kg), the measuring range of the tester is 0 to 200 N and 0 to 698.2 mmHg (depending on the calibration factor S), which is enough to test the validation of the tourniquets.

Design files summary
The design files are summarized in Table 2.
The tourniquet tester is cylindrical in shape and has an LCD screen at the top to display the measured values. The housing of the tester is made of hard thermoplastic material, and there are four assembly holes for round buttons made of flexible   thermoplastic polyurethane (TPU) material. The tester housing is 3-D printed and can be customized if necessary, which means that different sizes of housings can be made to suit different test conditions. See (Fig. 5). File 1. Amplifier mounting: This component is part of the amplifier assembling. To hold the amplifier in place, M2.5 bolts and nuts are used to secure the amplifier HX711 to the Amplifier mounting and then inserted the part into the notches reserved in the cylinder. File 2. Cap: top part of the housing, LCD is mounted on it. There are two holes on the side for the power supply. File 3. Flat: disk-shaped part for the load cell. It is the direct interface with Tournbutton-M. File 4. Thigh inside cylinder: the main part of the housing. Most electronic parts should be mounted on this part. File 5. Tournbutton-M: the button is made from hard material, for measuring. The cylinder part should be long enough so that it has enough space to be pushed down.  Tournbutton-T: button made of TPU material, mounted on the inside of the cylinder. It should be thick and flexible enough so that the tourniquet has enough space to generate pressure. See (Fig. 6).

Bill of materials summary
The full bill of materials can be downloaded from the Open Science Framework [80] containing links to suppliers but is summarized in Table 3. All costs are in Canadian dollars.

Build instructions
First, acquire all of the components in the BOM shown in Table 3. For the 3-D printed components detailed in Table 2, 3-D print using the setting summarized in Table 4.

Assembling the electronic components
Connect the HX711 amplifier to the load cell. For more reliability, solder the wires on the load cell to the amplifier. Then, use the jump wires to connect the LCD and amplifier to the Arduino Uno board as shown in Fig. 7. The wiring is further detailed in Table 5. Build the unit tester housing.
Once all the design files are 3-D printed, the assembly process is as follows with the steps shown in Fig. 8. Put the flat into the groove inside the cylinder.   5. Use M2.5 bolts and nuts to fix the amplifier on the 3-D printed amplifier part, and then insert the part into the reserved notch inside the cylinder.
6. Use M2.5 bolts and nuts to fix the Arduino U no board to the four posts on the cylinder. Only three of them have holes for fixing and one is for support only.
7. Use M2.5 bolts and nuts to fix the LCD on the cap, and then cover the cap, with the holes reserved on the cap aligned with the power and USB slots of the Arduino board.
8. Insert the TPU button into the reserved hole on the cylinder. The assembly is complete.
Because of the use of 3-D printing technology, the design supports changing the size to suit different situations. The cap and the cylinder are connected using a snap fit, which makes installation easy and saves mounting materials such as bolts and nuts.
3. To assemble the calibration platform (optional The calibration platform makes the calibration of the tester easy but is not necessary.

Operation instructions
Before testing the tourniquet, calibration is required. Due to the nature of the sensor, the calibration consists of two parts, Calibration of the load cell (Force reading) Calibration of the tourniquet tester unit as a whole (Pressure reading)

Calibration of the force measurement
Initially, the load cell can be calibrated using known weights. Known weights can be placed on the tournbutton-M while the tester is placed flat. This method has limitations on the weight that can be applied. Alternatively, a calibration device may be employed, which consists of a wooden plate, a linear rail, bolts and nuts, and other 3-D printed parts as Fig. 9 shows. Once assembled, the tester is placed beneath the 3-D printed platform and connected to a computer. By manipulating the weights on the platform, data can be obtained.
To get the calibration factor of the force measurement, an open-source calibration code [80] is utilized to acquire this factor. To apply this calibration code, first, it needs to be uploaded to the Arduino board through Arduino IDE software. There are instructions to be followed in the code including [81]: 1: Set up the tester and start the sketch without a weight on the tester. 2: Once readings are displayed place the weight on the scale. 3: Press +/-or a/z to adjust the calibration_factor until the output readings match the known weight. 4: Use this calibration_factor on the unit tester sketch.
The example code assumes pounds (lbs). In this project, 100 g was used as the unit. Unit can be changed in the Serial.print ('' lbs"); line to 100 g. The parameter that can be obtained could be a positive or negative number, it depends on the way of implementing the load cell. The factor typically ranges between 1500 and 15000. It should be noted that this parameter may differ depending on the environment in which the load cell is installed, thus necessitating recalibration each time the load cell is reinstalled. See ( Fig. 10 and Fig. 11).

Calibrate the pressure parameter
In most medical and clinical settings, the pressure generated by the tourniquet is recorded in mmHg. As the sensor in the unit tester is a load cell, it can only measure the force being applied, not the pressure. Therefore, the force measurement is converted into pressure, following:  where P is pressure (pascal), F is force (N) and S is surface area (m 3 ). Equation (1) requires an area to convert force into pressure, which in this case is the effective surface area acting on the load cell. Various factors such as the force concentration, width of the tourniquet, the number of turns of the windlass, and the part of the tourniquet that touches the button can also influence the readings. To address this issue and obtain more reliable pressure measurements, the method used to measure occlusion pressure was adopted, which uses a manual blood pressure cuff (manual sphygmomanometer) as the calibrating unit [68,71]. These blood pressure cuffs are widely accessible globally. This approach is divided into two parts. First, determine the optimum position of the blood pressure cuff with reference to the load cell/ ''tournbutton-M" part. Second, determining the effective surface area parameter.
To determine the optimum position of the blood pressure cuff for further testing, a manual pneumatic blood pressure cuff was wrapped around the unit tester, and the tourniquet was applied to the cuff. The pressure generated by the tourniquet was determined by subtracting the original reading from the reading after applying the tourniquet. An infant-sized blood pressure cuff was used to ensure consistency in the measurement position, and five readings were taken at 60 mmHg, 100 mmHg, 140 mmHg, and 180 mmHg, respectively. The cuff was adjusted such that the pneumatic pipe aligns exactly at the center of the tournbutton-M at 0 degrees. The cuff was then rotated by 45 degrees, and the readings were recorded again. This was repeated 8 times from 0 degrees to 315 degrees. Fig. 12 shows the readings of pressure applied by the blood pressure cuff and the force recorded by the load cell of the tourniquet tester as the system was rotated (Fig. 13). The data in Fig. 12 shows similar trends for all the angles of rotation tested. To determine the fixed position of the blood pressure cuff for further testing and validation, the 0-degree angle is chosen, where the cuff is exactly on top of tournbutton-M. This was done because the readings showed a linear relationship between the pressure applied by the tourniquet and the force measured by the load cell at the particular position. Now, to find out the surface area the procedure is as follows: 1. Inflate the pneumatic blood pressure cuff to about 20 mmHg. Place the blood pressure cuff around the tester in the 0degree position.
2. Record the force readings at 10 mmHg increments while increasing the pressure from turning the windlass of the tourniquet.
3. The following equations are used to calculate the value of the surface area, S. First the pressure, P is determined: where c represents the reading on blood pressure cuff in mmHg, F represents the reading of force (N) on the tester. In equation (2), the initially applied pressure of 20 mmHg is subtracted from the pressure reading. The constant 133.32 is the conversion factor for converting the pressure from mmHg to Pascals. Measure the force readings at least three times to obtain stable values of force against the applied pressure. Calculate the average of the force measurements and further calculate the surface area parameter S for each of the pressure readings. Finally, the average of all the S values is considered the final calibration factor/surface area parameter. See (Table 6).
Put S in the equation in the code ''unit tester" [80]. Once calibration is complete, the tester is ready for measurement. The measurement process is shown in Fig. 14 and is as follows: 1. Plug in the power cord and the tester starts up.
2. Insert Tournbutton-M into the hole used for measurement. 3. Wrap the tourniquet around the tester, and make sure that the tourniquet is pressing the Tournbutton-M, and the Tournbutton-M stands vertically on the flat (for the force can distribute evenly).
4. Turn the tourniquet windlass and the corresponding value will appear on LCD.

Validation
To validate the calibration procedure, 1: Inflate air into the blood pressure cuff to about 20 mmHg. 2: Put the blood pressure cuff in between the tourniquet and the tester. 3: Test the tourniquet on the unit tester. Record the pressure on the tester and blood pressure cuff. 4: Compare the difference between these data.
If the readings do not have a large gap (standard), then it can be concluded that the unit tester is valid. Fig. 8 shows the validation data, where the plot is an average of 5 tests of blood pressure cuff readings vs the calibrated tourniquet tester readings. The results fit within a relatively straight line with the R 2 value of 0.99. The slope of the line is 1.07 to confirm that the blood pressure cuff measurements and the tourniquet measurements correspond to the same value. Thus, it can be confirmed that the tester unit is validated. The same procedure can be used in any distributed manufacturing facility making the testers. The testers can then be deployed to tourniquet manufacturers.
To demonstrate how open-source tourniquet testers could perform across a diverse ecosystem, five tourniquets of varying style from geographically-dispersed manufacturers were tested, including those from volunteer fabricators.See Appendix A1 for photographs of various tourniquet types and tests. These tests were performed according to the test procedure in the previous section The purpose of this test was to check whether the tourniquet sample could meet the requirements of use, i.e., whether the pressure data could exceed 300 mm Hg. In addition, the test results recorded the pressure at each turn of windlass (180 degrees) and the different pressure data caused by the different parts of the tourniquet on the Tournbutton -M. Performance tests to demonstrate the tourniquet tester was adequate for a range of tourniquet types is shown in Table 7. Then to demonstrate reproducibility of a distributed manufacturing of a tourniquet the Canadian parts sample manufactured in Poland was repeated four times. These results are shown in Table 8. As can be seen in Tables 7 and 8, first all tourniquets were able to apply the clinical maximum rates pressures without breaking. Table 7 also shows the placement of the buckle on the sensor has an impact on the measured pressure and it is recommended that the buckle is placed on the sensor directly to have reproducible values. There was, however, unsurprisingly a range of recommended turns needed to get this pressure from the different tourniquet designs. Lastly, as seen by Table 8, the distributedly manufactured tourniquets did indeed have variability. Thus, even the recommended number of turns changes. It is thus recommended that each distributedly manufactured tourniquet be tested and have the recommended number of turns be placed on the package or strapping material in permanent marker.

Limitations and future works
After testing, the load cell used is stable and has good linear characteristics. The user operation is simple and straightforward. Readings are visible directly on the screen on top of the tester, with no additional operation or instrumentation is required. Due to the simple design and principle, the tester is also inexpensive to produce. This device can thus perform  Table 1 can also be used for training and have features specifically for this (e.g., they look like limbs and have synthetic blood that is stopped when adequate pressure is applied. For the testing of tourniquets none of these features are necessary. Future work is needed to determine if the device described in this paper would also be adequate for training. Overall, the device met the design goals and is ready for laboratory testing at tourniquet manufacturing sites of any kind. There are, however, several areas of future work and ways the system can be improved. First, it should be pointed out that the open-source tourniquet tester is validated for research purposes and has not undergone medical device regulatory scrutiny that may be required in some jurisdictions. Second, there are some potential improvements that can be made on the device itself. The printing time, the plastic used, and the cost can all be further reduced by shrinking the size of the assembly. This can in part be done by making a dedicated open-source board for this application or coupling the output of the device to a relatively ubiquitous cell phone for obtaining the readings. Secondly, the snap-fit part can be damaged if it is repeatedly opened and closed roughly. Therefore, it is recommended to reduce the number of disassembles, but also future work can investigate the use of other materials, improving the toughness of the design. There are some obvious limitations. Due to the housing being fixed, there is a maximum measuring limitation, but it is not the limitation of the tourniquet. It is because the pressure generated by the tourniquet comes from the deformation or compressing of the tourniquet. If the tourniquet cannot deform, then the force applied to the windlass will transfer to the strap itself, but not apply tothe cylinder. The TPU parts will help this situation, but the limitation still exists. Another limitation comes from the load cell: it has a measurement range, and if the force goes beyond its limit, the data will be inaccurate. The maximum force able to be measured on the system as designed is 200 N, which is the measurement range limit of the load cell (20 kg). It should be pointed out, however, that the value does not (and should not) go above 500 mmHg (approximately 154 N). After this amount of force is applied the patient can be damaged [82]. The tester should be deployed with this value labeled as a warning. Future work can also investigate the potential to make this a stand-alone (zero power) device by incorporating solar photovoltaic cells into the housing design. Other research groups associated with distributed tourniquet manufacturers can test additional types of tourniquets.

Ethics statements
No human and animal subjects are used in this research.

Funding
This work was supported by the Western Frugal Biomedical Innovation Strategic Grant, Glia, and the Thompson Endowment.

Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.